Quick start
- Enter X and Y, for example 15 and 80.
- Read X% of Y, X as a percent of Y, and the percent change from X to Y.
- The extra lines show Y increased and decreased by X%.
Three questions, three formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 80 = 12 |
| X is what % of Y? | X ÷ Y × 100 | 15 of 80 = 18.75% |
| % change from X to Y | (Y − X) ÷ X × 100 | 15 to 80 = +433.3% |
Reverse percentages
A jacket costs 68 after a 15% discount. What was the original price? The sale price is 85% of the original, so divide: 68 ÷ 0.85 = 80. Adding 15% to 68 gives 78.20, which is wrong, because the 15% was taken from the larger original price.
Increases and decreases do not cancel
A price that rises 20% and then falls 20% ends lower than it started. 50 → 60 → 48. The fall is 20% of a bigger number.
Percent or percentage points?
If an interest rate moves from 4% to 5%, it rose by 1 percentage point. In relative terms, though, it rose by 25%, because 1 is a quarter of 4. Both are correct; they just measure different things.
Common questions
How do I find 20% of a number quickly?
Find 10% by moving the decimal one place left, then double it. 20% of 340 = 34 × 2 = 68.
How do I add VAT or GST to a price?
Multiply by 1 plus the rate as a decimal. 500 plus 18% is 500 × 1.18 = 590.
How do I work out a percentage score?
Divide marks gained by total marks and multiply by 100. 42 out of 60 is 70%.